The invention relates to a particle-optical lens arrangement, in particular for the particle-optical imaging of an object, to be imaged and positionable in an object area, into an image area, as well as a method employing such a lens arrangement, in particular, a method for device manufacture comprising a photolithographic step. In particular, the lens arrangement is provided for use in an electron beam projection lithographic system as well as for use in a method for device manufacture by means of electron beam projection lithography.
The SCALPEL method (Scattering with Angular Limitation in Projection Electron-beam Lithography) is known as a method which employs a beam of electrons for imaging and exposing a radiation sensitive layer. This process is described in the white book xe2x80x9cSCALPEL: A Projection Electron-Beam Approach to Sub-Optical Lithographyxe2x80x9d, Technology Review, December 1999, by J. A. Liddle, Lloyd R. Harriott, A. E. Novembre and W. K. Waskiewicz, Bell Laboratories, Lucent Technologies, 600 Mountain Avenue, Murray Hill, N.J. 07974, USA. The entire disclosure of said document is incorporated herein by reference. Furthermore, U.S. Pat. Nos. 5,079,112, 5,130,213, 5,260,151, 5,376,505, 5,258,246, 5,316,879 as well as European patent applications nos. 0,953,876 A2 and 0,969,326 A2 relate to the SCALPEL process. The entire disclosures of the above-mentioned patent documents are likewise incorporated herein by reference.
A conventional imaging lens arrangement with two focusing lenses which can be employed for this purpose is described herein below with reference to FIG. 1.
FIG. 1 schematically shows a mask support 1 carrying an object, namely structures 3 which are provided in the form of a mask and are to be imaged, such that said structures are disposed in a focal plane of a focusing magnetic lens 5, i.e., they are spaced apart from the magnetic lens by a distance corresponding to the focal length of the magnetic lens.
The lens 5 images the object plane with the pattern 3 to be imaged to infinity, and a further magnetic lens 7 is provided for imaging the pattern imaged by the lens 5 onto a substrate surface 9 which is spaced apart from the lens 7 by a distance corresponding to the focal length thereof. Between the two lenses 5 and 7, there is provided an aperture stop 11 which is spaced apart from the lens 5 by a distance corresponding to the focal length thereof and from the lens 7 by a distance corresponding to the focal length of the lens 7. Particle beams 12, 13 traversing the mask outside of the structures 3 to be imaged traverse the mask substantially straightly such that they pass through a so-called xe2x80x9ccrossover pointxe2x80x9d disposed in the center of the aperture stop 11, so that these beams are imaged onto the substrate surface 9. The structures 3 to be imaged are defined by a material on the mask support 1, which material scatters the electrons comparatively strongly, so that beams impinging on such a stronger scattering structure 3 are deflected from their original direction, as it is shown in FIG. 1 for a beam 14. Accordingly, this beam cannot be deflected by the lens 5 to the crossover point in the aperture stop 11, so that it is absorbed by the aperture stop 11 and thus not imaged onto the substrate 9 either.
In order for the mask 3 to be imaged onto the substrate 9, it is not necessary to particle-optically illuminate the entire mask at once. Rather, it is also possible to illuminate at any time merely a subfield of the mask and to move said subfield over the mask area in scanning fashion. A subfield is outlined in FIG. 1 by central and peripheral beams 12, said subfield being positioned centrally on an optical axis 15 of the lenses 5 and 7. A subfield which is offset in the mask plane in respect of the optical axis 15 by a distance M is outlined by central and peripheral beams designated by reference numeral 13.
It has been found that, in the above-described lens arrangement, imaging characteristics satisfying higher demands are not achievable, in particular for subfields which are deflected from the optical axis.
It is an object of the present invention to propose a particle-optical imaging lens arrangement which enables a reduction of aberrations.
In particular, it is an object of the invention to propose a lens arrangement which enables the reduction of aberrations for particle beams which extend outside of an optical axis of components of the lens arrangement.
Moreover, it is an object of the invention to propose a method for manufacturing miniaturized devices which enables the device to be manufactured with increased precision.
According to a first aspect of the invention, a lens arrangement is provided for the particle-optical imaging of an object, to be imaged and positionable in an object area, into an image area, the lens arrangement comprising a first focusing lens device and a second focusing lens device as well as a deflection lens device. Here, the object area and the image area may at first be of any shape given by the image. However, in practice, an approximation of at least one of the areas to a planar shape is strived at in order to enable, first, a simple configuration of the object or/and the image area and, second, to reduce aberrations which result, among others, from the fact that the object is not exactly positioned in the intended object area or a substrate to be exposed is not exactly positioned in the intended object area.
The first focusing lens device provides a field which has a focusing effect on the imaging particles, so that at least a subfield of the object area is imaged into a subfield of an intermediate image area of the lens arrangement. The second focusing lens device, too, provides a field having a focusing effect, namely for imaging at least the subfield of the intermediate image area into a subfield of the image area.
In order to reduce aberrations of the first and/or the second focusing lens device, the lens arrangement comprises a deflection lens device for providing a field having a deflecting effect on the imaging particles in the region of the intermediate image area. The field of the deflection lens device having a deflecting effect causes the intermediate image to tilt before it is imaged by the second focusing lens device into the image area. Accordingly, aberrations which result into a tilt of the image area relative to a nominal image area can be compensated for. In particular, these aberrations are such which are referred to as image field curvature.
A field having a focusing effect is to be understood in this connection as a field which substantially does not deflect a suitably selected central beam and which deflects decentral beams extending with increasing distance from the central beam and parallel thereto towards the central beam, the angle at which the decentral beams are deflected towards the central beam also increasing with increasing distance of the diffracted beam from the central beam. This focusing effect need not be produced, at a given point in time, in the entire effective range of the fields of the first and the second focusing lens devices, respectively. Rather, it suffices if a partial section of the fields of the focusing lens device has such a focusing effect on the particles traversing said partial section. The effect which the focusing lens device has on the imaging particles is thus comparable to that which a convex lens has in light optics.
Preferably, the first or/and the second focusing lens device provides a field which comprises a magnetic or/and electric field which is substantially axially symmetric in respect of such a central beam of a bundle of beams. As an alternative or in addition thereto, the first or/and the second focusing lens device may comprise two or three axially spaced apart field arrangements in order to achieve the focusing effect, wherein the two field arrangements may comprise dipole or/and quadrupole field arrangements or combinations thereof and, together, produce the focusing effect. Generally, by positioning several field arrangements successively in beam direction, which field arrangements as such do not produce non-rotationally symmetric fields, a lens effect is achievable which corresponds to that of a round lens.
The deflecting field of the deflection lens device deflects particles traversing the same substantially into the same direction. Therefore, the field of the deflection lens device is in particular not axially symmetric. Preferably, the field of the deflection lens device is mirror-symmetric to a plane into which a central beam of a bundle of beams of the imaging particles extends. In particular, the field of the deflection lens device is a magnetic or/and electric dipole field which is oriented transversely to a direction of propagation of the imaging particles. The field of the deflection lens device thus has an effect on the imaging particles which is comparable to that of a wedge lens in light optics.
The field of the deflection lens device is effective in a certain region around the intermediate image area extending out of the intermediate image area, i.e., the area into which the object area is imaged by the first focusing lens device.
It is the characteristic of the intermediate image area that partial beams which emerge from the object area at one location, however, at different angles, traverse the intermediate image area substantially at a common location likewise at different angles, with beams emerging from different locations of the object area likewise traversing the intermediate image area at different locations.
The intermediate image area of the first focusing lens device thus differs from the diffraction area of the first focusing lens device which is positioned between the intermediate image plane and the first focusing lens device as such. Because it is the characteristic of the diffraction area that beams which emerge from the object area at different locations, but at the same angle, traverse the diffraction area at the same locations, but at different angles. Here, beams emerging from the object area at different angles also traverse the diffraction area at different locations.
Furthermore, a diffraction area allocated to the second focusing lens device is disposed between the intermediate image plane and the second focusing lens device. In this second diffraction area, beams which impinge on the image area at the same angle meet at the same location, but at different angles. The diffraction area of a focusing lens is also commonly referred to as Fourier area.
Preferably, an aperture filter is provided in the diffraction area of the first or/and the second focusing lens device for absorbing beams which traverse the object area at an angle exceeding a predetermined angle.
Preferably, the lens arrangement is provided for imaging a subfield of the object area into the image area, wherein the location of the subfield is variably adjustable likewise to a subfield of the object area. In particular, a distance of the subfield from a predetermined axis is adjustable, said axis comprising, for example, a central optical axis of the first or/and the second focusing lens device. It has been found, in this respect, that the deflection lens device is particularly suited to compensate for effects produced by the image field curvature caused by aberrations of the first or/and second focusing lens device. To this end, a field strength of the deflection lens device is preferably adjusted such that it changes proportional to the distance of the subfield from the predetermined axis. The deflection lens device causes the subfield imaged into the image area to be tilted and can be adjusted such that the image area in the region of the currently imaged subfield is of approximately planar shape and thus all subfields of a planar object area can be imaged successively in time substantially into a planar image area.
In this respect, it is in particular also provided for that the strength of the fields of the first or/and the second focusing lens device is changed quadratically dependent on the distance of the subfield from the predetermined axis. As a result, a malfocus caused by an image field curvature can be compensated for such that the images of a planar object area are substantially sharply imaged into a planar image area as well.
According to a second aspect, the invention also provides for a method for device manufacture, such devices being preferably highly miniaturized devices, such as micro-mechanical structures or integrated circuits. As far as integrated circuits are concerned, a mask includes a circuit pattern which corresponds to a single layer of the circuit to be formed on a suitable substrate, for example, a silicon wafer. In order to image the pattern onto a target area, also referred to as die, of the substrate, the latter is first covered with a radiation sensitive layer, also referred to as resist. Subsequently, the radiation sensitive layer is exposed or irradiated in that the pattern of the mask is imaged by means of charged particles, for example, electrons or ions, onto the radiation sensitive layer. The radiation sensitive layer is then developed and either the irradiated or exposed or the non-irradiated or unexposed regions of the irradiated layer are removed. The remaining structure of the radiation sensitive layer is then used as a mask, for example, in an etching step, an ion implantation step, a material deposition step or the like.
According to the invention, a pattern defined by a mask is imaged onto a particle-sensitive substrate by means of the above-described lens arrangement in a photolithographic step of the method.
Exemplary embodiments of the invention will be described below with reference to the accompanying drawings, wherein